Heating Options for a London Home Extension: Radiators, UFH, and Heat Pumps

The simplest heating approach for a new London home extension is to extend the existing central heating system (hot water central heating from the existing gas boiler) into the new extension by adding a new radiator or radiators on the existing pipework circuit. This is appropriate where: the existing boiler has sufficient spare output capacity to heat the new extension (most modern condensing gas boilers are oversized for the existing house — a 25kW boiler in a 3-bedroom London terrace typically has 4-8kW of spare capacity, which is more than sufficient to heat a single-storey rear extension of up to 20m²); the extension layout accommodates conventional radiators without compromising the open-plan design (in an open-plan kitchen-dining space with bifold doors, radiator placement is often restricted to under a fixed window or on the side wall — this is acceptable where the heating load of the extension is modest and the insulation standard is high); and the connection of the new circuit to the existing pipework does not require the existing boiler to be upgraded. Radiator sizing for a London extension: the heat loss of a new Part-L-compliant extension (cavity wall construction with 100mm PIR fill, Part L glazing, flat or pitched roof to Part L standard) is approximately 25-40 W/m² in London's climate zone — significantly lower than the 50-70 W/m² heat loss of a typical Victorian terrace room. A 20m² extension (open plan kitchen-dining space) may require only 500-800W of radiator output, which is achievable with a single large panel radiator (e.g., a 1000mm × 600mm double convector). Where underfloor heating is preferred for the extension floor but the existing heating system is a conventional high-temperature gas boiler (flow temperature 70-80°C), a mixing valve (e.g., a Honeywell/Resideo 3-way mixing valve) must be installed to reduce the flow temperature to the UFH manifold to approximately 40-50°C. This allows UFH to be added to the extension while the existing radiators in the rest of the house continue to run at their normal higher temperature.

Underfloor heating (UFH) is the most popular heating choice for a new London extension — particularly for an open-plan kitchen-dining space or an extension with a polished concrete, stone, or engineered timber floor finish. UFH eliminates radiators from the space, provides even warmth at floor level, and is invisible in use. Wet underfloor heating (hydronic UFH — water-bearing pipe in a screed): the most efficient long-term UFH option for a ground-floor extension. The UFH pipe (16mm PEX-a or polybutylene) is laid in a serpentine or spiral pattern on top of the insulation layer at 100-200mm centres, connected to a manifold on the wall, and covered by a 65mm anhydrite or sand-cement screed. The manifold controls the flow temperature (35-45°C from the mixing valve for UFH; the boiler continues to serve the rest of the house at its normal temperature). Wet UFH is connected to the existing gas boiler or can serve as the primary heated zone for a new ASHP if one is being installed. Cost: £70-£110 per m² supply and install for a new ground-floor extension (pipe, manifold, screed). Electric underfloor heating (electric mat or cable): suitable for bathrooms and smaller areas, but significantly more expensive to run than wet UFH as a primary heating system for a large space (electricity cost approximately 3× gas cost per kWh at 2025 tariffs). For a 20m² open-plan extension used daily as the primary living space, running cost of electric UFH can be £300-£600 per year more expensive than equivalent wet UFH linked to a condensing gas boiler. Electric UFH is appropriate as a supplement in tiled areas (bathroom, utility room within the extension) and for small areas (under-stair WC, boot room) where the wet pipe circuit would be disproportionate. Cost: £35-£65 per m² supply and install for a bathroom electric mat. UFH commissioning requirement: all wet UFH systems must be commissioned before the floor finish is laid — this involves filling the system, checking for leaks at all manifold and pipe connections, and running through a heat cycle. Building Regulations require that the system is handed over with commissioning documentation.

The UK government's trajectory toward phasing out new gas boiler installations (proposed ban on new gas boilers in new-build homes from 2025 — now deferred to approximately 2028-2035 depending on updated policy position as of mid-2025) means that many London homeowners are considering whether a home extension is the right moment to upgrade from a gas boiler to an air source heat pump (ASHP). Key considerations for ASHP suitability in a London extension context: extension insulation standard: a new extension built to Part L of Building Regulations has a higher insulation standard than the existing house — the extension element is suitable for heat pump heating from the outset. However, the existing house must also be insulated to a sufficient standard for the ASHP to operate efficiently — a heat pump in a poorly insulated Victorian terrace (solid walls, single-glazed windows, no cavity insulation) will have a low coefficient of performance (COP) and high running costs. The existing house needs at minimum: loft insulation (300mm+ mineral wool or equivalent); cavity wall insulation where the wall construction permits; and ideally solid wall insulation if no cavity exists. Radiator sizing for ASHP flow temperature: ASHPs operate at lower flow temperatures (40-55°C) than gas boilers (65-80°C). At lower flow temperatures, a larger radiator is needed to emit the same heat output (approximately 2.5× the area of a standard gas-boiler radiator at the same room heat loss). In practice, this means the existing radiators in the rest of the house may need to be replaced with larger models when an ASHP is installed — unless the existing radiators are already oversized (which they sometimes are in older London Victorian properties). The new extension is an opportunity to size the radiators or UFH system correctly for ASHP flow temperatures from the outset. Boiler Upgrade Scheme (BUS) grant: a grant of £7,500 is currently available (as of September 2026) for eligible homeowners installing an MCS-certified ASHP in a property with an EPC rating of D or above. This significantly reduces the net upfront cost of an ASHP installation (total cost £8,000-£18,000 before grant; £500-£10,500 net). The BUS grant is available through an MCS-certified installer — not through the homeowner directly. Installing an ASHP alongside a new extension avoids the disruption of installing the heat pump separately later, and the heat pump can be right-sized to account for both the existing house and the new extension heat loss.

Part L of the Building Regulations (Conservation of Fuel and Power) applies to all new extensions in England and sets minimum thermal performance standards for the new extension envelope (walls, roof, floor, windows, and doors). Part L compliance for a new extension includes: maximum U-values for the extension envelope: external walls: 0.28 W/m²K. Flat or pitched roof: 0.18 W/m²K (flat) / 0.15 W/m²K (pitched). Ground floor (new concrete slab with insulation): 0.22 W/m²K. New glazing (windows and bifold/sliding doors): 1.4 W/m²K for the overall unit. Note: these are the backstop (minimum) values — many specification systems achieve better than these values in practice. Heating system efficiency (Part L): where a new boiler is installed as part of the extension project, it must be a high-efficiency condensing gas boiler (minimum 92% seasonal efficiency, ErP A rating or above). Where the existing boiler is being extended to serve the new extension without being replaced, Part L does not mandate replacement of the existing boiler on a domestic extension project — only on new dwellings. Thermostatic control: the new extension heating zone must have independent thermostatic control — either a thermostatic radiator valve (TRV) on the new radiator, or a room thermostat for the UFH zone controlled through the UFH manifold. This is a Part L requirement. Hot water cylinder insulation (where applicable): any new or replacement hot water cylinder must have a minimum of 50mm of factory-applied insulation (factory-insulated cylinders are standard on all modern unvented cylinders and hot water cylinders sold in the UK). The Building Control inspector will check Part L compliance at the first-fix and at completion stages of the extension project.